Keys for a Long-Standing Scientific Conundrum: The Phylogenetic Position of Stylophorans within Deuterostomes

Stylophorans (cornutes, mitrates) are Palaeozoic calcite-plated, marine deuterostomes. They consist of two regions: a delicate appendage, and an asymmetrical, flattened body. Since more than 40 years, their precise phylogenetic placement within deuterostomes remains highly controversial, as there is no agreement on the interpretation of several key anatomical features. The recent discovery of putative echinoderm-like fossils in the Early Cambrian of China (vetulocystids) was presented as a strong evidence supporting a basal placement of stylophorans within deuterostomes, either as hemichordate-like, basal-most members of the echinoderm stem-group, or as early chordates (calcichordates). These two closely-related scenarios are based on (1) the absence of pentaradial symmetry in stylophorans, (2) the interpretation of their appendage as a tail, and (3) the interpretation of some cornutan orifices as gill slits. A critical review of these arguments is proposed here. (1) Pentaradial symmetry was frequently, and independently lost in Palaeozoic echinoderms: its absence in stylophorans is thus not necessarily original and could be related to their epibenthic, free mode of life. (2) The distal portion of the stylophoran appendage consists of two sets of opposite movable cover plates articulated to one series of underlying ossicles: this morphology is not compatible with an interpretation of the stylophoran appendage as a closed, tightly sutured organ (stem or tail); it rather suggests close similarities with various echinoderm feeding appendages (arms, brachioles). (3) Holes in the body wall of some cornutes are interpreted as gill slits; however, similar orifices (e.g., epispires) occur in various echinoderms, in which they are interpreted as respiratory structures, not as gill openings. In conclusion, the three main arguments suggesting a basal placement of stylophorans within deuterostomes (asymmetry, tail, gill slits) can be falsified by a detailed examination of the fossils. Stylophorans more likely correspond to derived echinoderms, which have secondarily lost their radial symmetry.